Acquired antibiotic resistance is that which evolves in populations of susceptible commensal and pathogenic bacteria colonizing and infecting hosts that are under antibiotic treatment or prophylaxis. Acquired antibiotic resistance can result in treatment failure and contribute to transmissible or primary antibiotic resistance. The research proposed in this application will be devoted to understanding, in a quantitative and predictive way, the genetic, bacterial, host factors and population dynamic processes responsible for the evolution of acquired resistance in populations of bacteria infecting uncompromised mammals treated with single and multiple antibiotics. Towards this end, we will develop and analyze the properties of mathematical and computer simulation models of the within-host population dynamics of antibiotic treatment and the evolution of resistance and perform in vitro and in vivo (laboratory mouse) experiments with a capsulated E. coli (018:K1 :H7). In these experiments, we will estimate the parameters of these models, evaluate the reality of the assumptions behind their construction and test the validity of the predictions made from the analysis of their properties. The goals of this investigation are to; (1) Elucidate the conditions (dosage levels and treatment regimes) under which selection will favor the evolution of resistance in uncompromised mammals infected with antibiotic susceptible bacteria and treated with single antibiotics, multiple antibiotics and antibiotics for which clinical resistance requires multiple mutations. (2) Evaluate the contribution of post antibiotic effects (delays in the resumption of normal growth of antibiotic exposed bacteria after antibiotics are no longer at inhibitory concentrations) to the evolution of acquired resistance in populations of bacteria infecting antibiotic treated mammals. (3) Determine the contribution of elevated mutation rates to evolution acquired antibiotic resistance and the conditions under which antibiotic-mediated selection will result in the evolution of genes that augment mutation rates, mutator genes. The proposed research directed at these goals is in part, motivated by an academic interest in the mechanisms of adaptive evolution in bacteria. This research is also motivated by its direct utility to the health sciences and, in particular, to facilitate the design and evaluation of clinically effective antibiotic treatment protocols that minimize the likelihood of acquired antibiotic resistance evolving in the target population of bacteria.